1. Field of the Invention
The present invention relates to audio signal processing and, in particular, to mixing audio signals.
2. Description of the Related Art
A common problem in many areas of audio recording, audio production, and audio coding is the mixing of a number of input audio channels to generate a different number of output audio channels. The change in the number of channels can be motivated by, e.g., a difference between the number of audio tracks (channels) of the recording device and the number of channels supported by the playback system or a difference between the number of channels of the distribution medium (e.g., CD, DVD, radio) and the number of channels supported by the playback system.
One example is FM radio where audio is usually delivered as a two-channel stereo signal, but many receivers have only one playback channel (one loudspeaker). In such case, the two stereo channels are typically down-mixed to form a single mono audio signal for playback. Another example of down-mixing occurs when the multiple (e.g., five) channels of an input signal conforming to a surround audio format, e.g., 5.1 surround, are down-mixed for playback, e.g., using a two-channel stereo system.
In addition to down-mixing (i.e., where the number of output channels is less than the number of input channels), mixing of audio signals may involve up-mixing (i.e., where the number of output channels is greater than the number of input channels). An example of up-mixing is the generation of a three-channel output signal (e.g., left, right, and center) from a two-channel (e.g., left, right) input signal.
Both down-mixing and up-mixing should be performed using processes that degrade the spatial image quality as little as possible. In a conventional mixing process, each output channel is derived from a weighted summation of the input channels in the time domain, where “panning laws” or “matrixing schemes” are applied to determine the weight of each input channel for each output channel.
By operating on signals in the time domain, conventional mixing techniques are applied to the full bandwidth of the audio channels. One drawback of such mixing techniques is a possible loss of spectral energy and/or loudness when some of the spectral components of the input audio channels are not aligned in phase. Such phase differences commonly occur, for instance, in multiple-microphone recordings, when each microphone has a different distance to the sound source. As a result, different phases are introduced in the microphone signals. In the extreme case of a 180-degree phase shift between two input channels of the same amplitude, an output channel composed of the input-channel sum would have zero energy since the input signals would cancel each other. In a realistic scenario, the signal energy and/or loudness can significantly drop for some spectral components in the output channels due to the misaligned signal phases. This energy/loudness loss is audible as a coloration effect.
Coloration effects were studied to a large extent in the framework of the “mono-compatibility” problem at the time when two-channel stereo sound was widely introduced in the 1950's and 60's. See, e.g., F. K. Harvey and E. H. Uecke, “Compatibility problems in two-channel stereophonic recordings,” 13th AES meeting, preprint 197, October 1961, the teachings of which are incorporated herein by reference. The goal of these studies was to find ways to create a two-channel stereo signal that would largely maintain the perceived loudness over frequency when both channels are mixed to mono. Some sample solutions are to impose a phase shift of +45 and −45 degrees on the left and right audio signals, respectively, before summation or to use coincidence microphones for the recording (where both microphones have the same distance from the sound source). Although these techniques tend to reduce the coloration effect for particular situations, there are many audio processing applications in which these techniques either cannot be applied and/or will not provide satisfactory results.